In many applications, communications systems are equipped with more than one communications port to allow data to be transferred either to or from the communications system. FIG. 1 illustrates a representative communications system 104 with four (4) communications ports (COM1 108, COM2 112, COM3 116, and COM4 120). Note that COM2, COM3, and COM4 are not shown in detail, as they have the same configuration as COM1. Ports COM1–COM4 allow the use of different communications mediums (for example, modems, frame relays, modbus, SCADA). The purpose of multiple ports is to allow simultaneous access to the communication system by more than one user. These users can either input data to the communications system 104 or obtain data from the communications system 104.
It is within the prior art to assert, a Data Terminal Ready (“DTR”) signal by the Central Processing Unit (CPU) on to the communications ports via DTR1 (132), DTR2, DTR3, and DTR4. (DTR2–DTR4 not shown). In this way the DTR signal is passed to the user's communications device 180. This DTR signal indicates that the communication system 104 is ready for the connected user communication device 180 to read or write data to the communication system.
Typically, a communications system has an owner or administrator that is responsible for testing, maintaining, updating, or performing other administrative functions for the communications system. The administrator would use COM1 (108), COM2 (112), COM3 (116), or COM4 (120) to remotely access the communications system 104. Certain administrative tasks such as updating the communications system instruction set should be performed while the administrator has exclusive control of the communications system 104. One way to obtain exclusive control is to remove the communications system 104 from service or physically remove the communications connections from the other communications ports. For some applications it is not efficient or possible to physically isolate the communications system to gain exclusive control of the communications system for tasks such as an instruction update, periodic testing, or problem troubleshooting. For example, it is often not efficient or even possible to physically isolate a data acquisition system that aggregates and communicates measurements from remote or unmanned locations.
It is possible that while the administrator is performing administrator tasks, another user (User 1) can also access the communications system 104 through a communications port not being used by the administrator. Depending on the communications device 180 being used by User 1, it is possible for a connection to the communications system to be made even if a DTR signal 184 is not received by the communication device 180. Thus, the CPU 124 in the communication system may not be able to block incoming communication requests on the ports not being used by the administrator by merely withholding the DTR signal (such as DTR1 132 for COM1) from those other ports.
Depending on the activities that User 1 is performing within the communications system, the tasks being executed by the administrator may not function properly. Simultaneous access to the communications system by User 1 and the administrator during the execution of an administrative task requiring exclusive control of the communications system may cause the memory in the communications system to become corrupted and the communications system to malfunction. In addition, if User 1 is obtaining data while the administrator is performing administrator tasks, the data provided to User 1 may become corrupted and not be accurate. In some cases, the corruption to the data may be subtle so that it is not readily apparent that the data is unreliable.